In truth, it's a mundane line (3)
I believe the answer is:
rut
'a mundane line' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'in truth it's' is the wordplay.
'in' indicates a hidden word.
'RUT' can be found hidden inside 'truth ts'.
Can you help me to learn more?
(Other definitions for rut that I've seen before include "predictable course" , "Does get attention during this" , "Road depression, difficult to get out of" , "animal passion" , "Fixed (and tedious?) course" .)